FIX session msg vs session tags

Update, here’s an example of a session-related business message: 35=3

The “content” of FIX data is surrounded by session related data, including session-Messages and session-Tags. As a beginner, I was constantly confused over the difference between session-Messages vs session-Tags

A) There are only a few (I guess 5 to 10) session message types, such as heart beat, logon/logout, request for resend…
** All the types — session msg subtypes or app msg subtypes — show up under Tag 35.

B) In ANY (session/app) Message, there are (3 to 30) session-related header tags such often routing-related, such as (34)seqNum, (10)checksum, (8)beginString, (9)bodyLength, (52) timeStamp, encryption. I’d say even the all-important (35)MsgType tag qualifies as session tag.

Most of the standard header tags are optional.

In fact, application data are “surrounded” by session data. App data are the stars of the show, surrounded by a huge “supporting crew” —
– A session msg’s fields are 100% session-related;
– An app msg is not free of session information. It is partly a session msg — due to those session tags.
– If you snoop the conversation, most messages are session messages — heartbeats
– every business data exchange starts with and ends with session messages — logon and logout.

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